Measuring devices are used frequently in automation and process control technology for measuring process variables, such as flow (e.g. flow rate), fill level, pressure and temperature, or some other physical and/or chemical process variable of a process. The present assignee produces and sells, among others, measuring devices, under the marks Micropilot and Prosonic, which work according to travel-time measurement methods and which serve for ascertaining and/or monitoring a fill level of a medium in a container. In travel-time measuring methods, by way of example, ultrasonic waves are transmitted from a sound transducer, or microwaves, respectively radar waves, are transmitted via an antenna or are guided on a waveguide protruding into the medium. These transmitted waves are reflected on the surface of the medium and received back, following a distance-dependent travel time of the signal, and are then called “wanted”, or “useful”, echo-signals, or waves. From the travel-time, taking into consideration the known propagation velocity of the particular transmitted waves, the fill level of the medium in a container can be calculated.
Travel-time measuring methods are divided, fundamentally, into two ascertainment methods. Thus, a first travel-time measurement method is the time-difference measurement method, which ascertains the duration of the travel-time of a transmitted, high-frequency, pulse signal on a path over which it travels. Another widely used ascertainment method involves determining the frequency difference of a continuously transmitted, high-frequency signal, whose transmission frequency is, for example, continuously changed with time, relative to the reflected, received, high-frequency signal (FMCW—Frequency Modulated Continuous Wave). In the following, only the pulse travel-time method will be discussed; however, the method of the invention is equally applicable to the other travel-time methods, such as e.g. FMCW.
The transmitted measuring signals form, with the received, wanted echo signals, a total measurement signal, which can, under real measurement conditions, also contain interference echo signals. These interference echo signals arise from various causes, for example:                Reflection on objects installed in or on the container;        multi-path propagation (retro-reflections) and multi-mode propagation;        dispersion of the propagated waves;        foam- and accretion-formation of the medium;        filling and emptying procedures;        reflection characteristics of the medium;        low dielectric constant of the medium;        humidity in the container;        turbulent surface of the medium.        
In the current state of the art, there are various approaches toward the goal of removing these interference echo signals from the total measurement signal, since interference echo signals can make difficult the evaluation and ascertaining of fill level. For instance, they can cover the wanted echo signal.
In European Patent EP 1 069 438 A1, a method and apparatus are proposed, which, independently of interference signals, and, particularly, independently of multiple reflections or multi-mode propagation, enable a highly accurate fill level measurement. Via a special manner of proceeding, at certain fill level values, a correction value is ascertained from the difference of the amplitude distance value and the phase distance value, and is stored. Between two correction values of the certain fill level values, an interpolation of the values is performed. By these correction values, any fill level can be ascertained highly accurately, independently of multiple reflections and multimode propagation.
Another approach is described in German Patent DE 43 27 33 C2 and concerns a method for measuring fill level using a travel-time measuring device, wherein the interference signals are corrected by means of subtracting, from the total signal, an ascertained intensity value of the first encountered interference signal.
Additionally, in published international application WO 03/016835 A1, a method for evaluating measurement signals of a measuring device working according to a travel-time principle is described, wherein a currently recorded measurement curve is compared with reference signal data. In the comparison of the reference signal data to the currently recorded measurement curve, a correction factor can be determined from the time shifting of corresponding interference and wanted signals. The correction factor is then used to ascertain the sought fill level in cases where the wanted signal of the fill level is not present or can not be evaluated.
The methods and devices of the above-described applications, while dedicated to the removal of interference signals from the measurement signal, all have the problem, that they can not react to changes of process conditions in the container and influencing the measurement signal, or to changes of measurement method and measurement performance of the measuring device.
In published European patent application EP 0 961 106 A1, a fill level measuring device for continuous measurement of fill level of a fill substance in a container is described.
In this patent application (FIG. 4 with description), such interference echo signals are ascertained, for example, in a limit curve, and stored. This limit curve is ascertained on the basis of a measurement in empty container and represents, thus, the so-called “empty echo-function”, to which an additional, constant offset of the amplitude values is added. In the method of ascertaining the fill level, only those values are used in the current echo function, which lie above this limit curve. In the description (FIGS. 2 and 3), also discussed is the problem of technical, process conditions, which arise during a measuring of travel time of waves in a container. The problem solution proposed in the patent application involves providing, in addition to the fill level measuring device, four limit value switches at different heights in the container, for enabling a correction, or calibration, of the limit curve to the technical, process changes in the container. This integration of limit value switches has the disadvantages that the additional limit value switches increase costs for the total measurement installation and introduce additional disturbing elements on the container wall, such as can influence the measurement signal by reflected, interference echo signals.